One crustacean came to dominate the Cambrian seas: the trilobite. From 526 MYA these armored arthropods diversified to 17,000 species.
Trilobites were the cockroaches of their age. These shallow-benthic marine arthropods were common in a variety of environments, including tidal flats, with an amazing diversity of shapes and sizes.
By 520 MYA, trilobites were 2/3rds of all marine species. Most trilobites were less than 10 cm, but one, Paradoxides, at some 60 centimeters, was a prodigious paradox.
In typical radiation, trilobites accommodated themselves to many lifestyles: plankton-feeding swimmers, seabed scavengers, filter feeders, and predators. Olenidae, a family of trilobite, evolved a symbiotic relationship with sulfur-eating bacteria, and were thereby able to tolerate the oxygen-poor, sulfur-rich seabed at the time. Trilobites were one of the 1st animals to evolve a complex digestive system.
475 MYA, somewhat mysteriously, many trilobites abruptly vanished: declining to 1/3rd of the sea species, as mollusks, corals, and other stationary filter feeders rose. Stagnation – brought on by a strong thermocline – has been suggested, as has a sea level drop followed by a rise, though how this might have provoked trilobite decimation is unclear. Anyway, something begat a drawn-out decline of trilobites.
Glaciation at the end of the Ordovician wiped out the dominant trilobite group, the Ibex Fauna, while another group, the Whiterock Fauna, swam through the extinction event unscathed. Adaptive tolerances to variable conditions paid off.
The coming of jawed fish put a crunch on trilobites. Still they hung on. A long run on life’s stage – 340 million years – came to a close for trilobites in the excruciating mass extinction event at the end of the Permian, 250 MYA.